Physiological rhythm is central to establishment of interactive synchrony between mothers and infants and subsequently interactive synchrony is the foundation for development of infant regulation. Few studies have associated developing infant circadian rhythm and maternal entrainment with maternal-infant interactive synchrony and subsequent infant regulation. Feeding difficulties, disrupted sleep-wake pattern, and altered maternal-infant interaction are characteristic of dysregulated infants. The purpose of the proposed research is to characterize the l trajectory of infant circadian rhythm development, examine early synchrony of maternal and infant circadian rhythm and interaction, and predict infant regulation based on rhythm and interactive synchrony. Primary Aims include: (1) Determine the longitudinal pattern of maternal and infant circadian rhythm timing and strength as measured by cycle acrophase, amplitude, and cosinor fit (R2) of actigraphy recordings performed at 4, 8, and 12 weeks infant age. (2) Explore longitudinally environmental influences (light, home routine) on entrainment of circadian rhythm as measured by maternal and infant salivary melatonin circadian rhythm assessed at 4, 8, and 12 weeks infant age. (3) Examine the longitudinal pattern of maternal- infant physiological synchrony defined as the separate correlations between mother and infant circadian rhythm (CR-CORR) parameters (acrophase, amplitude, and cosinor fit) derived from actigraphy recordings performed at 4, 8, and 12 weeks infant age. (4) Assess the prediction of (a) maternal-infant interactional synchrony (CO-ACT), as measured by proportion of coordinated gaze, vocalization, and touch at 12 weeks, and (b) infant physiological and emotional regulation as measured by Infant/Toddler Symptom Checklist (ITSC), Laboratory Temperament Assessment Battery (Lab-TAB), and Brief Infant Sleep Questionnaire (BISQ) at 32 weeks infant age, using maternal-infant physiological synchrony (CR-CORR) as predictor. Forty healthy mother-infant pairs will be studied using a longitudinal design with times of measure at 4, 8, 12, and 32 weeks infant age. At 4, 8, and 12 weeks circadian rhythm of activity will be determined in the home using actigraphy recordings, combined with measurement of environmental light, and salivary samples will be collected serially throughout a 24-hour day to assess melatonin rhythm. At 12 weeks face-to-face affect synchrony (CO-ACT) will be coded from a 5 minute video recording of mother-infant interaction. At 32 weeks infant regulation will be assessed using the Laboratory Temperament Assessment Battery (Lab-TAB), Infant/Toddler Symptom Checklist (ITSC), and Brief Infant Sleep Questionnaire (BISQ) and maternal sleep and wake disturbance, depression, and fatigue as well as home routine will be assessed. Circadian rhythm will be derived from cosinor analysis. Cosinor parameters (acrophase, amplitude, cosinor fit) will be correlated (CR-CORR). Maternal-infant physiological synchrony (CR-CORR) will be used to predict interactive synchrony at 12 weeks and infant regulation at 32 weeks.